Adhesive sheet and method for peeling same

ABSTRACT

Provided is an adhesive sheet including a substrate sheet and a soluble adhesive layer with an island or stripe pattern disposed on at least one surface of the substrate sheet, wherein each adhesive region has a circumscribed circle diameter of 0.1 mm to 10 mm for the island pattern, or has a stripe width of 0.1 mm to 10 mm for the stripe pattern. This adhesive sheet can retain an adhesive force of the adhesive layer until just before releasing and can be readily released from the adherend at any time without applying of excessive stress to the adherend.

TECHNICAL FIELD

The present invention relates to an adhesive sheet and a method of releasing the same.

BACKGROUND ART

Adhesive sheets have been widely used in various applications, such as adhesion and temporary fixing of articles. Typical adhesive sheet includes a substrate sheet provided with an adhesive layer containing an adhesive component thereon. The adhesive sheets are classified into two types, i.e., “a bondable adhesive sheet” in which the adhesive layer of the substrate sheet itself is bonded to an adherend and “a transferable adhesive sheet” in which the adhesive layer is transferred onto an adherend or second substrate sheet and then the original substrate is released to impart adhesiveness to the adherend or second substrate sheet.

Adhesive sheets are known that include substrate sheets and adhesive layers having predetermined printed patterns. For example, PTL 1 (JP2010-174148A) discloses an adhesive product provided with an adhesive layer formed by intermittently disposing a dotted adhesive on a substrate for the purpose of fixing paper articles such as envelopes. This literature proposes use of an acrylic adhesive containing an acrylic copolymer to print the pattern of the adhesive layer precisely and efficiently on the substrate through, for example, screen printing and gravure printing. PTL 2 (JPH08-333556A) discloses a transferable adhesive sheet having an adhesive layer consisting of an aggregate pattern of dotted adhesive, the transferable adhesive sheet being used for bonding by transfer to, for example, plastic, metal, and ceramic products, and discloses, for example, acrylic, rubbery, and silicone adhesives.

Adhesive sheets have been sometimes used in processes of manufacturing semiconductor packages to improve the handling properties of thinned printed wiring boards. For example, PTL 3 (JP2014-7315A) discloses a process of manufacturing a semiconductor package. The method comprises bonding a heat-resistant film consisting of a self-adhesive elastomer layer on a heat-resistant support layer to a printed wiring board; and releasing the heat-resistant film after mounting of semiconductor chips, reflow treatment, sealing with resins and curing treatment. This document also proposes a substantially dotted or striped self-adhesive elastomer layer.

CITATION LIST Patent Literatures

PTL 1: JP2010-174148A

PTL 2: JPH08-333556A

PTL 3: JP2014-7315A

SUMMARY OF INVENTION

As disclosed in PTLs 1 to 3, various adhesive sheets are known. Since the adhesive layer retains a predetermined adhesive force, excessive stress may be applied to an adherend to release the adhesive sheet from the adherend. Although such excessive stress would not cause a significant problem if the adherend is a paper product as described in PTL 1 or a plastic product as described in PTL 2, excessive stress will lead to a critical problem if the adherend or application are adversely affected by the excessive stress (for example, precision parts and electronic parts). In the case that the adhesive sheet is bonded to reinforce a printed wiring board followed by various processes, such as mounting of chips, solder reflow and compression molding in the manufacture of a semiconductor package, the disconnection or release of the wiring layer occurs by excessive stress applied on the printed wiring board when the adhesive sheet is released, resulting in a decrease in yield.

In this respect, it is conceivable that an adhesive layer is composed of a material the adhesive force of which decreases by heat or ultraviolet irradiation and then the adhesive force is lost through the heat or ultraviolet irradiation during a releasing operation, to facilitate release of the sheet. In the case of the ultraviolet irradiation, applicable substrate sheets or adherends are limited to those capable of transmitting ultraviolet light. Another problem in this procedure is that the adhesive force of the adhesive layer is unintentionally reduced by the heat or ultraviolet irradiation to a level less than a required adhesive force before the releasing step. For example, if the adhesive layer contains thermally expandable microcapsules, the thermoplastic resin constituting the shell of capsules is softened by heat and the capsules expand and foam into micro-balloons, resulting in a decrease in the adhesive force of the adhesive layer. Unfortunately, these thermally expandable microcapsules are not durable to heating processes such as the solder reflow (for example, 260° C. or more).

The present inventors have now found that an adhesive sheet including a soluble adhesive layer with a predetermined size of island or stripe pattern can retain the adhesive force of the adhesive layer until just before releasing and can be readily released from the adherend at any time without applying of the excessive stress to the adherend.

Accordingly, an object of the present invention is to provide an adhesive sheet that can retain the adhesive force of the adhesive layer until just before releasing and can be readily released from the adherend at any time without applying of the excessive stress to the adherend.

According to one embodiment of the present invention, an adhesive sheet is provided, the adhesive sheet comprising a substrate sheet and a soluble adhesive layer with an island or stripe pattern disposed on at least one surface of the substrate sheet, wherein each adhesive region has a circumscribed circle diameter of 0.1 mm to 10 mm for the island pattern, or has a stripe width of 0.1 mm to 10 mm for the stripe pattern.

According to another embodiment of the present invention, a method of releasing the adhesive sheet or the substrate sheet from the adherend to which the adhesive sheet is bonded is provided, the method comprising the steps of:

-   -   impregnating the gaps of the island or stripe pattern of the         soluble adhesive layer with an alcoholic solution capable of         dissolving the soluble adhesive layer to dissolve or soften the         soluble adhesive layer; and     -   releasing the adhesive sheet or the substrate sheet from the         adherend after the soluble adhesive layer is dissolved or         softened.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating one embodiment of an adhesive sheet of the present invention.

FIG. 2 is a schematic top view of the adhesive sheet shown in FIG. 1.

FIG. 3 is a view for explaining that a releasing liquid penetrates into gaps in the pattern of the soluble adhesive layer.

FIG. 4A is a view illustrating an exemplary shape of an adhesive region.

FIG. 4B is a view illustrating another exemplary shape of the adhesive region.

FIG. 5 is a view for explaining a pitch circle diameter (PCD) in a dotted pattern.

FIG. 6A is a view illustrating an example of an island pattern.

FIG. 6B is a view illustrating an example of an island pattern composed of one or more clusters.

FIG. 6C is a view illustrating another example of an island pattern composed of one or more clusters.

FIG. 6D is a view illustrating another example of an island pattern composed of one or more clusters.

FIG. 6E is a view illustrating another example of an island pattern composed of one or more clusters.

FIG. 6F is a view illustrating another example of an island pattern composed of one or more clusters.

FIG. 7A is a process flowchart illustrating an exemplary use of an adhesive sheet in the manufacture of a semiconductor package.

FIG. 7B is a process flowchart illustrating an exemplary use of the adhesive sheet in the manufacture of a semiconductor package, including steps after the steps shown in FIG. 7A.

FIG. 7C is a process flowchart illustrating another exemplary use of the adhesive sheet in the manufacture of a semiconductor package, including steps after the steps shown in FIG. 7A.

DESCRIPTION OF EMBODIMENTS Adhesive Sheet

The adhesive sheet of the present invention is schematically illustrated in FIGS. 1 and 2. As shown in FIGS. 1 and 2, the adhesive sheet 10 of the present invention comprises a substrate sheet 12 and a soluble adhesive layer 14 provided on at least one surface of the substrate sheet 12. The soluble adhesive layer 14 is provided in an island or stripe pattern. Individual adhesive regions 14 a have a circumscribed circle diameter of is 0.1 to 10 mm in the case of the island pattern, or a stripe width of 0.1 to 10 mm in the case of the stripe pattern. In the present specification, the numerical range referred to by the expression “A to B” indicates from A or more to B or less. As described above, the soluble adhesive layer 14 is provided in the island or stripe pattern with a predetermined size; hence, the adhesive sheet retains the adhesive force of the adhesive layer until just before releasing, and can be readily released from the adherend at any time without applying of the excessive stress to the adherend. In other words, the soluble adhesive layer 14, employed in the adhesive sheet 10 of the present invention, can be dissolved or softened by immersing the soluble adhesive layer 14 in a soluble liquid (hereinafter referred to as a releasing liquid). Furthermore, the soluble adhesive layer 14 is provided in an island or stripe pattern with a predetermined size; hence, the soluble adhesive layer 14 can be completely impregnated with the releasing liquid (for example, an alcoholic solution). As schematically shown by dotted arrows in FIG. 3, this advantage is probably achieved based on the effective penetration of the releasing liquid L into the gaps in the pattern of the soluble adhesive layer 14 and the promotion of the contact with each adhesive region 14 a when the soluble adhesive layer 14 bonded to the adherend 20 (for example, a printed wiring board) is immersed in the releasing liquid L. As a result, the release of the sheet can be facilitated from the adherend 20. The release of the sheet from the adherend 20 may be spontaneously achieved by dissolution of the soluble adhesive layer 14, or mechanically achieved after the adhesive force is significantly reduced by dissolution or softening of the soluble adhesive layer 14.

Since the adhesive layer in a general adhesive sheet has a predetermined adhesive force as described above, excessive stress may be applied to the adherend to release the adhesive sheet from the adherend. The excessive stress applied to the adherend may cause quality matter of the adherend or the application (for example, precision parts and electronic parts). For example, in various processes, such as mounting of chips, solder reflow, and compression molding after bonding the adhesive sheet to reinforce a printed wiring board in the manufacture of a semiconductor package, excessive stress applied to the printed wiring board to release the adhesive sheet causes the disconnection or release of the wiring layer, resulting in a reduction in yield. In this respect, it is conceivable that the adhesive layer is composed of a material the adhesive force of which decreases by heat or ultraviolet irradiation and then the adhesive force is lost through heat or ultraviolet irradiation at a releasing operation to facilitate the release of the sheet. Such a procedure, however, has a problem in that the heat or ultraviolet irradiation before the release process unintentionally decreases the adhesive force of the adhesive layer to a level less than a required adhesive force. In contrast, in the adhesive sheet 10 of the present invention, the soluble adhesive layer 14 is completely impregnated with the releasing liquid, and the sheet can thereby be released without excessive stress to the adherend. Moreover, the soluble adhesive layer 14 is immersed in the releasing liquid immediately before the release of the sheet; hence, the adhesive force of the soluble adhesive layer 14 can be retained before the release, in other words, the release of the adhesive layer due to a decrease in adhesive force can be avoided before the releasing process. Accordingly, the sheet can retain the adhesive force of the adhesive layer before releasing and can be released from the adherend at any time.

In particular, thinning of a laminate is required in view of recent technical trends that employ fan-out wafer level packaging (FO-WLP) and panel level packaging (PLP). However, in the case of a thin laminate, a large local warpage of a build-up layer may occur when a substrate is removed from the laminate produced by, for example, a coreless build-up process. Such a large warpage of the build-up layer may cause the disconnection and separation of the wiring layer inside the build-up layer, and thereby decrease the reliability of connection in the wiring layer. In order to address the disconnection and separation of the wiring layer inside the build-up layer, a reinforcing sheet may be stacked on a laminate with an adhesive release layer therebetween. As a result, the laminate can be reinforced to an extent not causing a large local warpage, resulting in an improvement in the reliable connection in the laminate and the flatness (coplanarity) on the surface of the laminate. The present invention has an advantage in that the reinforcing sheet tightly adhering to the laminate with the soluble adhesive layer 14 therebetween can be efficiently released without applying of excessive stress to the laminate.

The adhesive sheet 10 may be either a bondable adhesive sheet or a transferable adhesive sheet. Accordingly, the adhesive sheet 10 may include a substrate sheet 12 having desired functions (e.g., a protective function and/or a reinforcing function) after the adhesive sheet 10 itself is bonded to the adherend, and as shown in FIGS. 7A (c) and (d), or the adhesive sheet may be used for transferring the soluble adhesive layer 14 to the second substrate sheet 12′ (e.g., a protective sheet or a reinforcing sheet) and releasing the initial substrate sheet 12 after transferring. In the latter case, the laminate having the soluble adhesive layer 14 transferred to the second substrate sheet 12′ is also included in the scope of the adhesive sheet of the present invention as a second adhesive sheet 10′.

The soluble adhesive layer 14 is provided in an island or stripe pattern on at least one surface of the substrate sheet 12. Two soluble adhesive layers 14 may be provided on the two surfaces of the substrate sheet 12. Individual adhesive regions 14 a have a circumscribed circle diameter in the range of 0.1 to 10 mm, preferably 0.1 to 5.0 mm, more preferably 0.1 to 2.0 mm in the island pattern, and a stripe width in the range of 0.1 to 10 mm, preferably 0.1 to 5.0 mm, more preferably 0.1 to 2.0 mm in the stripe pattern. Such a range can sufficiently maintain the adhesive force of the soluble adhesive layer 14 before immersion in the releasing liquid and promote the impregnation of the releasing liquid into the gaps in the pattern of the soluble adhesive layer 14, and thereby facilitate the release of the sheet from the adherend through, for example, a dissolution release.

The soluble adhesive layer 14 is preferably provided in an island pattern. The island pattern indicates that each adhesive region 14 a is surrounded by a non-adhesive region 14 b not having the soluble adhesive layer 14. Examples of the specific shape of each adhesive region 14 a constituting the island pattern include polygons and circles; in specific, polygons consisting of an intricate straight contour such as a star shape shown in FIG. 4A; and consisting of an intricate curved line contour such as an ameba shape shown in FIG. 4B. The island pattern is preferably a dotted pattern, and the shape of each dot may be typically a circle, or a shape close to a circle. The dot diameter defined as the circumscribed circle diameter of each dot constituting the dotted pattern is preferably 10 mm or less, more preferably 0.1 to 5.0 mm, further more preferably 0.1 to 2.0 mm. Such a configuration causes an increase in surface area of the soluble adhesive layer 14 and an enhancement of solubility, resulting in an improvement in releasability. In addition, the dotted pattern has a pitch circle diameter (PCD) of preferably 0.45 to 3.0 mm, more preferably 0.6 to 2.4 mm, further more preferably 0.8 to 2.0 mm. In the present specification, “a pitch circle diameter (PCD) of a dotted pattern” is defined to indicate a diameter (pitch circle diameter) of an imaginary circle (pitch circle (PC)) which can be formed by connecting the centers of three closest dots in the dotted pattern as shown in FIG. 5. The above range of the PCD can sufficiently maintain the adhesive force of the soluble adhesive layer 14 before immersion in the releasing liquid and promote the penetration of the releasing liquid into the gaps in the pattern of the soluble adhesive layer 14, thereby further facilitating the release of the sheet from the adherend.

The soluble adhesive layer 14 has a thickness of preferably 0.5 to 50 μm, more preferably 1.0 to less than 30 μm, further more preferably 1.0 to 20 μm, particularly more preferably 2.0 to 15 μm, most preferably 3.0 to 10 μm. The above range of the thickness allows the releasing liquid to rapidly penetrate into the gaps in the pattern of the soluble adhesive layer 14, resulting in an improvement in releasability of the adhesive sheet 10, as well as a decrease in indentation corresponding to the island or stripe pattern on the adherend. In particular, when mounting of chips, solder reflow and compression molding are performed after bonding an adhesive sheet to reinforce a printed wiring board in the manufacture semiconductor package, the indentation derived from the adhesive region 14 a may remain on the printed wiring board. However, the soluble adhesive layer 14 with a thickness of 7.0 μm or less has an advantage in that the indentation barely remains on the printed wiring board after the compression molding. In this respect, in the soluble adhesive layer 14 having a dotted pattern, a dot diameter of 0.7 mm or less and a thickness of the soluble adhesive layer 14 of 1.0 to 7.0 μm are particularly preferred from the viewpoint of more effective realization of both the reduction in indentation and the improvement in releasability.

The distance between the centers of the circumscribed circles in the adhesive regions 14 a is preferably larger than the mean diameter of the circumscribed circles, because a sufficient gap can be secured among the individual adhesive regions 14 a. From this point of view, the distance between the centers of the circumscribed circles in the adhesive region is preferably 0.1 to 20 mm, more preferably 0.2 to 10 mm, further more preferably 0.3 to 5.0 mm, particularly more preferably 0.4 to 2.0 mm. Such a range allows the releasing liquid to rapidly penetrate into the gaps in the pattern of the soluble adhesive layer 14, resulting in an improvement in releasability.

The island pattern may be composed of one or more clusters that form a polygonal, circular, annular, striped or grid pattern on the whole, and each of the clusters may consist of an aggregate of three or more adhesive regions. In other words, the island pattern (for example, a dotted pattern) has not only an embodiment where the adhesive regions are uniformly provided over the entire surface of the substrate sheet 12 as shown in FIG. 6A, but also an embodiment that wholly illustrates the polygonal pattern (e.g., a square as shown in FIG. 6B), the circular pattern as shown in FIG. 6C, the annular pattern as shown in FIG. 6D, the striped pattern shown in FIG. 6E, or the grid pattern as shown in FIG. 6F, where all these patterns are formed by clusters 18 each composed of an aggregate of three or more adhesive regions 14 a. In this manner, the island pattern has some advantages other than an improvement in appearance: For example, the adhesive region 14 a can be formed so as to avoid the areas that do not require higher adhesion strength and the areas where the indentation of the adhesive region 14 a tends to remain; the quantity of the soluble adhesive can be reduced per one adhesive sheet; and the soluble adhesive layer 14 can be efficiently dissolved because the releasing liquid rapidly penetrates into the region where the adhesive region 14 a is not formed.

The rate of the adhesive region 14 a to the total surface area provided with the soluble adhesive layer 14 of the substrate sheet 12 is preferably 3 to 90 area %, more preferably 3 to 75 area %, further more preferably 5 to 60 area %, particularly more preferably 5 to 40 area %. In the present specification, “the total surface area provided with the soluble adhesive layer 14 of the substrate sheet 12” indicates the total area of the surface adjacent to the soluble adhesive layer 14 of the substrate sheet 12 if the soluble adhesive layer 14 is provided on one side of the substrate sheet 12, or the total area of two sides of the substrate sheet 12 if the soluble adhesive layer 14 is provided on two sides of the substrate sheet 12. The above range of the rate of the adhesive region 14 a can sufficiently educe the adhesive force of the soluble adhesive layer 14 before immersion in the releasing liquid, and promote the penetration of the releasing liquid into the gaps in the pattern of the soluble adhesive layer 14 and thereby further facilitate the release of the sheet from the adherend. However, any range may be employed in applications that require only a minimum adhesive force in the process for the purpose of simple fixation.

The soluble adhesive layer 14 has, needless to say, adhesiveness at room temperature, and can be dissolved or softened in the releasing liquid. Accordingly, the soluble adhesive layer 14 preferably contains a solution-soluble resin, for example, an acid-soluble resin or an alkali-soluble resin. The solution-soluble resin can be efficiently dissolved or softened through the contact with the releasing liquid, resulting in more effective release of the sheet from the adherend.

The solution-soluble resin is preferably an alkali-soluble resin for the following reason: It is preferred that the soluble adhesive layer 14 be not substantially dissolved in a neutral or acidic solution in the application to articles that may come in contact with the neutral or acidic solution in the manufacturing process, such as a printed wiring board. For example, in the case that the soluble adhesive layer 14 is applied to a printed wiring board, the use of resins (for example, water-soluble resins) that dissolve in a neutral solution may cause the dissolution of resins in a water washing process in the manufacturing process of the printed wiring board, or the degradation of resins due to adsorption of moisture in the air. The selection of resins that dissolve only in an acidic solution (that is, resins that does not dissolve in a neutral or alkaline solution) may cause a decrease in adhesive force of the soluble adhesive layer 14, because the acidic solution is used in many micro-etching and etching steps for formation of circuit, which are cleaning steps of the surface in the manufacturing process of a printed wiring board. However, it is understood that the resins that dissolve in the neutral or acidic solution can be used in the applications where the article or adherend does not come in contact with the neutral or acidic solution in the manufacturing process.

In particular, the alkali-soluble resin preferably includes a polymer containing carboxyl groups and/or phenolic hydroxyl groups. Such a polymer can particularly readily dissolve in an alkaline solution, and thereby can promote the dissolution of the soluble adhesive layer 14 and cause the release of sheet from the adherend in a shorter time. Examples of the polymer containing carboxyl groups and/or phenolic hydroxyl groups include acrylic resins containing carboxyl groups and phenol novolac resins containing phenolic hydroxyl groups. The acrylic resin adhesive can be synthesized through copolymerization of an acrylic monomer (for example, acrylic acid or methacrylic acid) having a carboxyl group and an unsaturated double bond in the molecule with ethyl acrylate or butyl acrylate. In the synthesis, the type and ratio of the acrylic monomer is adjusted to control the adhesive force of the soluble adhesive layer 14 and the solubility of the soluble adhesive layer 14 in the alkaline solution. The adhesive force of the soluble adhesive layer 14 and the solubility of the soluble adhesive layer 14 in the alkaline solution can also be controlled through addition of a resin which triggers a crosslinking reaction of the carboxyl groups (for example, an epoxy resin) to the acrylic resin containing the carboxyl groups. Since crosslinking of some of the carboxyl groups in the acrylic resin with a resin such as an epoxy resin causes an increase in the molecular weight, the adhesive force and the solubility in the alkaline solution are reduced although the heat resistance is improved. In contrast, use of the phenol novolac resin containing the phenolic hydroxyl groups as the alkali-soluble resin decreases the adhesive force of the soluble adhesive layer 14; hence, a tackifier, such as rosin, is preferably mixed to yield appropriate adhesiveness.

For a soluble adhesive layer 14 containing an alkali-soluble resin, the alkaline solution used as the releasing liquid is preferably a sodium hydroxide solution and/or a potassium hydroxide solution. These solutions have preferably a concentration of 0.5 to 50 wt %. Within such a range, the solution has high alkalinity and high solubility, and sodium hydroxide and/or potassium hydroxide is less likely to precipitate even in lower room temperature when the releasing liquid is used. Moreover, an organic material (e.g., ethanolamine) an aqueous solution of which exhibits alkalinity may be used alone or in combination with the above solution.

In order to shorten a dissolution time of the soluble adhesive layer 14, an organic solvent capable of dissolving the acrylic resin and/or the novolac resin (e.g., 2-propanol) may be added to the alkaline solution. The preferred concentration of this organic solvent is 5 to 50 wt % for 100 wt % of the alkaline solution. Such a range can desirably shorten the dissolution time and reduce the volatilization of the solvent during the process, resulting in facilitating the control of the concentration of alkaline material and an improvement in safety. The organic solvent is preferably alcohol, and preferred examples of the alcohol include 2-propanol, methanol, ethanol, and 2-butanol.

An appropriate amount of surfactant may be added to the alkaline solution. The addition of the surfactant can improve the impregnation and wettability of the solution to the resin, and thus further shortens the dissolution time in the soluble adhesive layer 14. Any type of surfactant may be added. For example, the water-soluble surfactant may be any of anionic, cationic and nonionic surfactants.

The substrate sheet 12 may be in any form, such as a sheet generally referred to, a film, a plate, and a foil. The substrate sheet 12 may be also a laminate of these sheets, films, plates, and foils. Moreover, for adjusting the adhesive force between the substrate sheet 12 and the soluble adhesive layer 14, the surface, to which the soluble adhesive layer 14 is applied, of the substrate sheet 12 may be preliminarily subjected to known surface treatment, such as polishing, application of a releasing agent, and/or plasma treatment.

According to a preferred embodiment of the present invention, the substrate sheet 12 is composed of preferably at least one resin of poly(ethylene terephthalate) (PET) and polyethylene (PE), and more preferably poly(ethylene terephthalate) (PET). In particular, when the adhesive sheet 10 is used as a transferable adhesive sheet, it is preferred that the substrate sheet 12 support the soluble adhesive layer 14 and transfer the soluble adhesive layer 14 to the adherend or the second substrate sheet separately provided. The substrate sheet 12 of the present embodiment is suitable for such use. The substrate sheet 12 as the transferrable adhesive sheet has a thickness of preferably 10 to 200 μm, more preferably 20 to 150 μm, further more preferably 25 to 75 μm.

According to another preferred embodiment of the present invention, the substrate sheet 12 is preferably composed of at least one selected from the group consisting of metal (preferably a metal plate or a metal foil), glass, glass epoxy resin, polyimide resin, and phenol resin (Bakelite). The substrate sheet 12 for disposable use is preferably composed of phenol resin (Bakelite) from the viewpoint of material cost, whereas the substrate sheet 12 for repeated use is preferably composed of metal from the viewpoint of material strength. Since all of these materials are heat-resistant, deterioration can be prevented in the heat treatment process. In the case that the adhesive sheet 10 is used as a bondable adhesive sheet, it is preferred that the substrate sheet 12 have a function of supporting the soluble adhesive layer 14, as well as a function as a reinforcing sheet for improving handling properties and preventing or restraining the warpage of the printed wiring board in the process of manufacturing the semiconductor package. The substrate sheet 12 of the present embodiment is suitable for such use. The substrate sheet 12 as a bondable adhesive sheet has a thickness of preferably 10 μm to 1 mm, more preferably 50 to 800 μm, further more preferably 100 to 600 μm from the viewpoint of strength retention of the substrate sheet 12 and ready handling for the substrate sheet 12.

In the case that the adhesive sheet 10 is used as a bondable adhesive sheet, the substrate sheet 12 preferably has a lower Vickers hardness than the adherend (e.g., the printed wiring board). This lower hardness causes the stress that may be generated during stacking and releasing to be appropriately relieved by warp of the substrate sheet 12 itself when the adhesive sheet 10 is stacked on or released from the adherend (e.g., a printed wiring board), resulting in effective prevention or restraint of the warpage on the adherend (e.g., a printed wiring board). The Vickers hardness of the substrate sheet 12 is preferably 2 to 99%, more preferably 6 to 90%, and further more preferably 10 to 85% of that of the adherend (e.g., a printed wiring board). The substrate sheet 12 has a Vickers hardness of preferably 50 to 700 HV, more preferably 150 to 550 HV, further more preferably 170 to 500 HV. In the present specification, the Vickers hardness is measured in accordance with the “Vickers hardness test” described in JIS Z 2244-2009.

For reference, the Vickers hardness HV of various candidate materials is exemplified as follows: sapphire glass (2300 HV), hard metal alloy (1700 HV), cermet (1650 HV), quartz (rock crystal) (1103 HV), SKH 56 (high speed tool steel, HSS) (722 HV), tempered glass (640 HV), SUS 440 C (stainless steel) (615 HV), SUS 630 (stainless steel) (375 HV), titanium alloy 60-types (64 titanium alloy) (about 280 HV), Inconel (heat resistant nickel alloy) (150 to 280 HV), S 45 C (carbon steel for machine structural use) (201 to 269 HV), Hastelloy alloy (corrosion resistant nickel alloy) (100 to 230 HV), SUS 304 (stainless steel) (187 HV), SUS 430 (stainless steel) (183 HV), cast iron (160 to 180 HV), titanium alloy (110 to 150 HV), brass (80 to 150 HV), and bronze (50 to 100 HV).

When the adhesive sheet 10 is used as a bondable adhesive sheet, the substrate sheet 12 has a spring limit value Kb_(0.1) in the range of preferably 100 to 1500 N/mm², more preferably 150 to 1200 N/mm², further more preferably 200 to 1000 N/mm² as measured in accordance with the repeated deflection test of JIS H 3130-2012. Such a range causes the stress that may be generated during stacking and releasing to be appropriately relieved by warping of the substrate sheet itself when the adhesive sheet 10 is stacked on or released from the adherend (e.g., a printed wiring board), resulting in effective prevention or restraint of the warpage on the adherend (e.g., a printed wiring board). Since the substrate sheet 12 warped when stacked or released can instantaneously return back to the original flat shape due to its resiliency, the flatness (coplanarity) of the adherend (e.g., a printed wiring board) can be more effectively maintained. In addition, the use of the tenacity and resiliency of the substrate sheet 12 can urge the substrate sheet 12 to which the releasing force is applied in the direction of the release (that is, in the direction away from the adherend (e.g., a printed wiring board)), resulting in further smooth release.

For reference, spring limit values Kb_(0.1) for various candidate materials are illustrated in Tables 1 and 2.

TABLE 1 Material (JIS Number) Tempering Kb_(0.1) SUS301CSP 1/2H 315 (stainless steel) 3/4H 390 H 490 EH 590 SHE(1) 650 SUS304CSP 1/2H 275 (stainless steel) 3/4H 335 H 390 SUS631CSP O 635 (stainless steel) 1/2H 635 3/4H 835 H 980 SUS632J1CSP 1/2H 1200 (stainless steel) 3/4H 1400 C1700 O 685 (beryllium-copper alloy) 1/4H 785 1/2H 835 H 855 C1720 O 735 (beryllium-copper alloy) 1/4H 835 1/2H 885 H 930 C1720M OM 390 (beryllium-copper alloy) 1/4HM 440 *mill-hardened material 1/2HM 540 HM 635

TABLE 2 Material (JIS Number) Tempering Kb_(0.1) C5210 1/2H 245 (phosphor bronze) H 390 EH 460 SH 510 ESH 560 C5240 H 430 (phosphor bronze) EH 510 SH 570 ESH 630 XSH 700 C7270 O 490 (nickel-tin-copper alloy) 1/4H 590 1/2H 635 H 685 EH 735 SH 785 C7270M OM 440 (nickel-tin-copper alloy) 1/4HM 490 *mill-hardened material 1/2HM 540 HM 590 EHM 685 XHM 785 C7701 1/2H 390 (nickel-zinc-copper alloy) H 480 EH 560 SH 620 C1990M 1/4HM 440 (titanium-copper alloy) EHM 590 *mill-hardened material

In the case that the adhesive sheet 10 is used as a bondable adhesive sheet, the substrate sheet 12 preferably has through-holes in the thickness direction. Before the adhesive sheet 10 is released from the adherend (e.g., a printed wiring board), the releasing liquid can penetrate into the gaps in the pattern of the soluble adhesive layer 14 though these through-holes provided in the substrate sheet 12, and the soluble adhesive layer 14 is more efficiently dissolved, resulting in a further improvement in releasability.

As shown in FIG. 7A (a) described later, the adhesive sheet 10 further preferably includes a protective film 16 stacked on the soluble adhesive layer 14, resulting in ready handling. The protective film 16 is composed of any material, preferably a resin. Preferred examples of the resin constituting the protective film include poly(ethylene terephthalate) (PET), polyethylene (PE), or a combination thereof, and more preferably poly(ethylene terephthalate) (PET). The protective film 16 has a thickness of preferably 10 to 40 μm, more preferably 15 to 30 μm. Since the protective film 16 protects the soluble adhesive layer 14 before the adhesive sheet 10 is used, the protective film is released from the adhesive sheet 10 in use of the adhesive sheet 10. Accordingly, the adhesive force (e.g., release strength) between the soluble adhesive layer 14 and the protective film 16 is preferably lower than the adhesive force (e.g., release strength) between the soluble adhesive layer 14 and the substrate sheet 12. This difference in the adhesive force allows the protective film 16 to be preferentially released to the substrate sheet 12, thereby the protective film 16 can be released readily and smoothly while stably maintaining the adhesion between the substrate sheet 12 and the soluble adhesive layer 14. In order to adjust the adhesive force between the protective film 16 and the soluble adhesive layer 14, the surface, on which the soluble adhesive layer 14 is applied, of the protective film 16, may be preliminarily subjected to surface treatment by a known procedure, such as polishing treatment, application of release agent, and plasma treatment.

Use of Adhesive Sheet in Manufacture of Semiconductor Package

The adhesive sheet 10 of the present invention can be applied to various adherends that require a desired adhesiveness, and used in any field, most preferably for reinforcing of printed wiring boards. FIGS. 7A to 7C illustrate steps for manufacturing a semiconductor package using the adhesive sheet 10 of the present invention. The steps shown in FIG. 7B may be carried out after the steps shown in FIG. 7A, or the steps shown in FIG. 7C may be carried out after the steps shown in FIG. 7A. In any case, the process shown in FIG. 7A includes the following steps: The adhesive sheet 10 provided with the substrate sheet 12, the soluble adhesive layer 14, and the protective film 16, in sequence, is prepared to be a transferable adhesive sheet (step (a)); The protective film 16 is released (step (b)); The soluble adhesive layer 14 is transferred by a roll lamination process to the second substrate sheet 12′ functioning as a reinforcing sheet (step (c)); and The substrate sheet 12 is released (step (d)). In these steps, the second adhesive sheet 10′ comprising the second substrate sheet 12′ to which the soluble adhesive layer 14 is transferred as described above is also categorized within the scope of the adhesive sheet, as a bondable adhesive sheet, of the present invention. The second adhesive sheet 10′ is bonded to the printed wiring board 20 p by a vacuum lamination process (step(e)). As shown in FIG. 7B, the resulting laminate may be immersed in an alkaline solution after the printed wiring board 20 p is subjected to solder paste printing (step (i)), and the second substrate sheet 12′ (reinforcing sheet) may be released (step (j)). In contrast, as shown in FIG. 7C, the resulting laminate may be immersed in an alkaline solution (step (i)), and the second substrate sheet 12′ (reinforcing sheet) may be released (step (j)) after being subjected to mounting of the semiconductor chip 22 (chip mounting) (step (f)), solder reflow (step (g)), and compression molding (step (h)). As shown in FIG. 7C, the semiconductor chip 22 may be sealed with the insulating resin 24 in the step (h). Since the second substrate sheet 12′ serves as the reinforcing sheet in any case, the reliability of connection and the surface flatness (coplanarity) on the printed wiring board 20 p can be enhanced, resulting in simplification in processes and an improvement in yield. Since the adhesive force of the soluble adhesive layer 14 can be maintained until the releasing step, an unintended release does not occur to cause no trouble in handling. In the final step, immersing in the alkaline solution allows the second substrate sheet 12′ to be readily released from the printed wiring board 20 p without applying of the excessive stress to the printed wiring board 20 p.

Release of Adhesive Sheet

The adhesive sheet 10 of the present invention is used to be bonded to any adherend 20. The following procedures are preferably performed for the release of the adhesive sheet 10 or the substrate sheet 12 from the adhered 20 to which the adhesive sheet 10 is bonded.

An alcoholic solution capable of dissolving the soluble adhesive layer 14 is penetrated into gaps in the island or stripe pattern of the soluble adhesive layer 14 to dissolve or soften the soluble adhesive layer 14. The penetration of the alcoholic solution into the gaps may be performed by bringing the whole or part of the adherend 20 to which the adhesive sheet 10 is bonded into contact with an alcoholic releasing solution through, for example, immersion. Examples of the alcohol contained in the alcoholic solution capable of dissolving the soluble adhesive layer 14 include preferably 2-propanol, methanol, ethanol, and 2-butanol, and most preferably 2-propanol from the viewpoint of high miscibility with water and superior releasability. The alcohol in the alcoholic solution has a concentration in the range of preferably 5 to 90 wt %, more preferably 10 to 70 wt %, further more preferably 20 to 50 wt %. Such a range allows the soluble adhesion layer 14 to be more effectively dissolved or softened, and causes the alcoholic solution to be hard to ignite due to the reduced alcoholic concentration and thereby improve the safety. The alcoholic solution preferably contains a basic substance, such as KOH, NaHCO₃, Na₂CO₃, NaOH, to control and stabilize the releasability, and the basic substance has a concentration of preferably 0.1 to 30 wt %, more preferably 0.5 to 20 wt %, further more preferably 1.0 to 15 wt % in the alcoholic solution. Accordingly, the alcoholic solution may contain the alkaline solution as described above.

After the soluble adhesive layer 14 is dissolved or softened, the adhesive sheet 10 or the substrate sheet 12 is released from the adherend 20. The release of the sheet from the adherend 20 may be spontaneously achieved by dissolution of the soluble adhesive layer 14, or mechanically achieved after the adhesive force is significantly reduced by dissolution or softening of the soluble adhesive layer 14.

Production of Adhesive Sheet

A soluble adhesive is provided, and an island or stripe pattern of the soluble adhesive can be printed on the substrate sheet 12 to form a soluble adhesive layer 14. The adhesive sheet 10 can thereby be produced. Since the pattern of the soluble adhesive layer 14 and the size and shape of the individual adhesive regions 14 a are substantially equivalent to a block design in printing, the desired soluble adhesive layer 14 is formed by appropriately modifying the block design. The printing may be performed using any known process, for example, dot pattern printing, screen printing, or gravure printing (intaglio printing). The gravure printing that can employ a roll to roll process is preferably used from the viewpoint of stable continuous printing and high mass productivity.

EXAMPLES

The invention will be described in more detail by the following examples.

Examples 1 to 3 (1) Production of Adhesive Sheet

A curing agent (1 wt %, TETRAD-C, available from Mitsubishi Gas Chemical Co., Ltd.) was added to a carboxyl group-containing acrylic resin (COPONYL™ N-2584, available from Nippon Synthetic Chemical Industry Co., Ltd.) to prepare a soluble adhesive containing a polymer having carboxyl groups. A predetermined dotted pattern shown in Table 3 of this soluble adhesive was gravure-printed on a poly(ethylene terephthalate) (PET) sheet to give an adhesive sheet. The gravure printing was carried out with an engraved printing block (a cylinder having a print width of about 700 mm and a diameter of 200 mm) in which a printed image of the dotted pattern satisfied the specifications: a dot diameter (i.e., a circumscribed circle diameter of individual dots), a pitch circle diameter (PCD), and the rate of adhesive region shown in Table 3 such that the total thickness of the substrate sheet and the soluble adhesion layer was 100 μm.

(2) Evaluation of Adhesive Sheet

The resultant adhesive sheet was subjected to evaluation of the following properties.

<Evaluation 1-1: Releasability (in Alkaline Solution)>

The releasability in an alkaline solution was evaluated according to a process flowchart shown in FIGS. 7A and 7B. In this process flow, a stainless-steel plate was stacked as a second substrate sheet 12′ on the surface provided with the soluble adhesive layer 14 of the adhesive sheet 10 (step (c)), and then the substrate sheet 12 was released from the laminate (Step (d)). The soluble adhesive layer 14 of the adhesive sheet 10 was transferred to the stainless-steel plate that is the second substrate sheet 12′. The second substrate sheet 12′ (the stainless-steel plate) is bonded to the printed wiring board 20 p with the soluble adhesive layer 14 therebetween to produce a laminate 26 for evaluation (step (e)). The laminate 26 was stationarily immersed in a releasing liquid (10 wt % sodium hydroxide solution) (step (i)). The time (maximum 18 hours) is measured that is required from the start of immersion to the release of the second substrate sheet 12′ (the stainless-steel plate) from the printed wiring board 20 p (step (j)), and classified into the following grades. The results are as shown in Table 3.

Grade AA: The soluble adhesive layer was dissolved and the stainless-steel plate was released within 30 minutes, resulting in a significant improvement in productivity due to a significantly high dissolution rate of the soluble adhesive layer.

Grade A: The soluble adhesive layer was dissolved and the stainless-steel plate was released within more than 30 minutes to one hour, resulting in an improvement in productivity due to a high dissolution rate of the soluble adhesive layer.

Grade B: The soluble adhesive layer was dissolved and the stainless-steel plate was released within more than one hour to three hours, resulting in a releasing rate sufficient in the case of no particular requirement for acceleration of processing.

Grade C: The stainless-steel plate was not released even after immersion for 18 hours.

<Evaluation 1-2: Releasability (in Alcoholic Solution)>

Example 2 was subjected to evaluation of the releasability as in Evaluation 1-1 except that the releasing liquid was a mixed alcoholic solution of 1.5 wt % potassium hydroxide and 25 wt % 2-propanol, instead of the alkaline solution. The results are shown in Table 3.

<Evaluation 2: Indentation Marks (after Compression Molding)>

The indentations marks after compression molding were evaluated according to the process flowchart shown in FIGS. 7A and 7C. In detail, the laminate for evaluation 26 was produced as in Evaluation 1 (steps (c) to (e)); the semiconductor chip 22 was mounted on the printed wiring board 20 p of the laminate 26 (step (f)); solder reflow is performed (step (g)); and compression molding was then performed at 175° C. under 8 MPa to give a laminate 28 for evaluation of the indentation marks (step (h)). This laminate 28 for evaluation of the indentation marks was stationarily immersed in a releasing solution (10 wt % sodium hydroxide solution) (step (i)); and the second substrate sheet 12′ (the stainless-steel plate) was released from the printed wiring board 20 p (step (j)). The indentation marks remaining on the printed wiring board 20 p were then observed with a microscope, and classified into the following grades. The results are shown in Table 3.

Grade A: The depth of indentation was less than 5 μm.

Grade B: The depth of indentation was 5 μm to 10 μm. This depth of indentation did not adversely affect the subsequent steps, but caused inferior appearance.

Unevaluated: Since the stainless-steel plate was not released even after immersion for 18 hours, the indentation marks was not evaluated.

Example 4 (Comparative)

An adhesive sheet was produced, and Evaluations 1-1 and 2 were performed as in Example 1, except that a printing cylindrical block which a printing image was not engraved was used to form a soluble adhesive layer through solid coating. The results are shown in Table 3.

TABLE 3 Example Example 1 Example 2 Example 3 4* Printed image Dot diameter (mm) 1.0 0.7 1.0 — engraved on Pitch circle diameter 1.80 1.26 1.80 — printing block (PCD) (mm) (cylinder) Rate of adhesive region 38 38 38 — (area %) (Calculated value based on printing block design) Soluble Pattern Dotted Dotted Dotted Solid adhesive layer printed Dot diameter (mm) 0.98 0.70 0.97 — Thickness (μm) 7 7 15 7 Evaluation Evaluation 1-1: B A A C Releasability (alkaline solution) (releasing liquid: 10 wt % NaOH solution) Evaluation 1-2: — AA — — Releasability (alcoholic solution) (releasing liquid: 1.5 wt % KOH + 25 wt % 2-propanol) Evaluation 2: A A B Unable Indentation marks to (after compression evaluate molding) *Comparative example 

1. An adhesive sheet comprising a substrate sheet and a soluble adhesive layer with an island or stripe pattern disposed on at least one surface of the substrate sheet, wherein each adhesive region has a circumscribed circle diameter of 0.1 mm to 10 mm for the island pattern, or has a stripe width of 0.1 mm to 10 mm for the stripe pattern.
 2. The adhesive sheet according to claim 1, wherein the soluble adhesive layer contains a solution-soluble resin.
 3. The adhesive sheet according to claim 2, wherein the solution-soluble resin is an alkali-soluble resin.
 4. The adhesive sheet according to claim 3, wherein the alkali-soluble resin includes a polymer containing carboxyl groups and/or phenolic hydroxyl groups.
 5. The adhesive sheet according to claim 1, wherein the soluble adhesive layer has a thickness of 0.5 μm to 50 μm.
 6. The adhesive sheet according to claim 5, wherein the soluble adhesive layer has a thickness of 3.0 μm to less than 10 μm.
 7. The adhesive sheet according to claim 1, wherein the soluble adhesive layer has the island pattern.
 8. The adhesive sheet according to claim 1, wherein the island pattern is a dotted pattern.
 9. The adhesive sheet according to claim 8, wherein the dotted pattern has a dot diameter of 0.7 mm or less, and the soluble adhesive layer has a thickness of 1.0 μm to 7.0 μm.
 10. The adhesive sheet according to claim 8, wherein the dotted pattern has a pitch circle diameter (PCD) of 0.45 mm to 3.0 mm.
 11. The adhesive sheet according to claim 1, wherein the distance between the centers of circumscribed circles in the adhesive region is greater than the mean value of the circumscribed circle diameters, and is in a range of 0.1 mm to 20 mm.
 12. The adhesive sheet according to claim 1, wherein the island pattern is composed of one or more clusters that form a polygonal, circular, annular, striped or grid pattern on the whole, and each of the clusters consists of an aggregate of three or more adhesive regions.
 13. The adhesive sheet according to claim 1, the rate of the adhesive region to the total surface area provided with the soluble adhesive layer of the substrate sheet is 3 to 90 area %.
 14. The adhesive sheet according to claim 1, wherein the substrate sheet is composed of at least one resin of poly(ethylene terephthalate) (PET) and polyethylene (PE).
 15. The adhesive sheet according to claim 1, wherein the substrate sheet is composed of at least one selected from the group consisting of metal, glass, glass epoxy resin, polyimide resin, and phenol resin.
 16. The adhesive sheet according to claim 15, wherein the substrate sheet has through-holes in the thickness direction.
 17. The adhesive sheet according to claim 1, further comprising a protective film stacked on the soluble adhesive layer.
 18. The adhesive sheet according to claim 17, wherein the protective film is composed of poly(ethylene terephthalate) (PET) and/or polyethylene (PE).
 19. A method of releasing the adhesive sheet or the substrate sheet from an adherend to which the adhesive sheet according to claim 1 is bonded, comprising the steps of: impregnating the gaps of the island or stripe pattern of the soluble adhesive layer with an alcoholic solution capable of dissolving the soluble adhesive layer to dissolve or soften the soluble adhesive layer; and releasing the adhesive sheet or the substrate sheet from the adherend after the soluble adhesive layer is dissolved or softened. 